In the old times you would get the ID of the user record in the Servoy repository. This was changed because people wanted to be able to manage their own identifiers, wherever they may come from (for example LDAP). So basically the idea is that YOU provide a uid when you create a user (see the last parameter).
Since you do not supply an ID, Servoy server falls back to the old mechanism of giving you the repository ID (an integer). In developer, there is no repository database, so you get a UUID.